APPLIED PHYSICS LETTERS
VOLUME 85, NUMBER 25
20 DECEMBER 2004
Guiding 1.5 µm light in photonic crystals based on dielectric rods
Solomon Assefa,a) Peter T. Rakich, Peter Bienstman, Steven G. Johnson, Gale S. Petrich,
John D. Joannopoulos, Leslie A. Kolodziejski, Erich P. Ippen, and Henry I. Smith
Research Laboratory of Electronics, Center for Materials Science and Engineering,
Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
(Received 11 May 2004; accepted 25 October 2004)
Photonic-crystal structures consisting of dielectric rods were designed, fabricated, and optically
characterized. The combination of the high refractive-index-contrast GaAs/ AlxOy material system
with electron-beam lithography enabled the fabrication of structures suitable for the optical
propagation of 1.5 µm light. Experimental transmission spectra were obtained for structures
consisting of a two-dimensional array of rods and line-defect waveguides. Optical measurements
confirmed the presence of a photonic band gap, as well as band gap guidance in the line-defect
waveguide. A two-stage coupling scheme facilitated efficient optical coupling into the line-defect
waveguide. © 2004 American Institute of Physics. [DOI: 10.1063/1.1840107]
Photonic crystals are anticipated to have impact on
large-scale photonic integrated-circuits by allowing the creation of compact and efficient devices such as waveguides,
splitters, and filters.1,2 Photonic crystals are dielectric structures having a periodically-modulated index of refraction.3,4
A waveguide is created by introducing a line-defect within
the photonic crystal. While the surrounding periodic arrangement of dielectric material provides a photonic band gap, the
line-defect functions as a waveguide by localizing certain
frequencies within the photonic band gap.5
Although three-dimensional photonic crystals provide
complete control for guiding light, two-dimensional (2D)
slab structures are simpler to fabricate using planar processing tools. Slab photonic crystals provide band gap confinement in the plane, and index confinement in the vertical
direction.6,7 Previous efforts have almost exclusively focused
on photonic crystals consisting of a triangular lattice of holes
in dielectric slabs, which have a band gap for TE-like
modes.8–10 This paper reports the design, fabrication, and
characterization of the “inverse” structure, a square lattice of
dielectric-rods in air. The structure has the unique feature of
being a disconnected topology that guides light. Also, the
dielectric rod structure is fundamentally different, compared
to the hole-based structures, due to the high aspect-ratio and
the band gap for TM-like polarization.
In the present work, the photonic crystals are composed
of a 2D array of high-index dielectric rods that reside on a
low-index layer, as shown schematically in Fig. 1(a). Each
dielectric rod consists of a 500 nm thick layer of high-index
GaAs, epitaxially bonded to an 800 nm thick layer of lowindex AlxOy. An additional 700-nm-thick AlxOy spacer layer
separates the dielectric-rod array from the underlying GaAs
substrate. For the photonic crystal, each rod has a diameter of
285 nm and is placed in a square lattice with a period of 500
nm. A linear waveguide is created by reducing to 235 nm the
diameter of a row of rods, forming a line-defect within the
photonic crystal, as seen in Fig. 1(a). The surrounding 2D
array of larger diameter GaAs rods establishes a 2D photonic
band gap (PBG), thereby confining the light within the
smaller diameter defect-row waveguide. In the direction nora)
Electronic mail: Solomona@mit.edu
mal to the plane of periodicity, light is index-confined in the
defect rods since each GaAs layer is between low-index
AlxOy and air.
To create the photonic-crystal waveguide devices, a
GaAs/ Al0.9Ga0.1As heterostructure was grown by gas-source
molecular-beam epitaxy on a GaAs substrate. Then, 300 nm
of SiO2 was sputter-deposited onto the III–V substrate. The
photonic crystals were defined with scanning-electron-beam
lithography using polymethylmethacrylate (PMMA) resist.
The PMMA was developed in a 1:2 solution of methylisobutyl-ketone (MIBK) and isopropanol (IPA). Next, a 35
nm thick nickel film was evaporated onto the samples and a
liftoff process was performed using a heated bath of
1-methyl-2-pyrrolidone (NMP). The nickel was used as a
hard mask to transfer the pattern into the SiO2 by reactiveion etching (RIE) in a CHF3 plasma. The nickel mask was
removed in a wet nickel etchant.
Using the SiO2 mask, the dielectric rods were created by
etching the GaAs and the underlying Al0.9Ga0.1As to a total
depth of 1.5 µm in a BCl3 plasma. The III–V etching step
was difficult due to the small feature size, the greater than
5:1 aspect ratio, and the desire to achieve a straight sidewall
profile with low sidewall roughness. Using a low dc bias and
low pressure during the RIE process, an etch rate of
70 nm/ min and 55 nm/ min was obtained for the GaAs and
Al0.9Ga0.1As layers, respectively.
Upon completion of the etching, each sample was
cleaved to create smooth input and output facets for optical
testing. The Al0.9Ga0.1As was transformed into the low-index
AlxOy layer using a wet thermal oxidation process. Steam
FIG. 1. (a) Schematic illustrating a 2D array of dielectric rods, conventional
dielectric input/output waveguides, and a line-defect waveguide. (b) Sideview scanning-electron microscope (SEM) image of a photonic crystal after
the final RIE and oxidation steps.
0003-6951/2004/85(25)/6110/3/$22.00
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© 2004 American Institute of Physics
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Appl. Phys. Lett., Vol. 85, No. 25, 20 December 2004
Assefa et al.
6111
FIG. 2. (a) Top-view schematic of a two-stage coupling
structure. (b) Side-view SEM image of the structure
after the final RIE and oxidation steps.
from a H2O bath, heated to 90 °C, was introduced into the
oxidation furnace using N2 as a carrier gas. An oxidation
temperature of 435 °C resulted in an oxidation rate of
⬃40 nm/ min. As the oxidation time required for the aforementioned structures is short, the oxidation process is
reaction-rate-limited (linear regime). Figure 1(b) shows a
scanning-electron microscope (SEM) image of a bulk
photonic-crystal device after fabrication.
Light can be coupled directly from a fiber to a rod-based
photonic-crystal waveguide by encapsulating the structure
with a low index polymer.11 However, a carefully designed
modal transformation is required to achieve efficient coupling from a conventional high-index-contrast input waveguide to a photonic-crystal defect-row waveguide.12,13 As reported in the literature, a gradual taper transition may offer a
potential mechanism to couple light from conventional
waveguides into photonic crystal waveguides.14,15 Without a
proper coupling design, the transition to the defect-row
waveguide was found to suffer from Fabry–Perot reflections,
resulting in transmission that is dependent on frequency and
the length of the photonic-crystal waveguide.
To facilitate coupling from a conventional high-indexcontrast waveguide into the photonic-crystal line-defect
waveguide, a two-stage coupling scheme was employed, as
shown schematically in Fig. 2(a). The forward propagating
optical mode in the conventional dielectric waveguide was
adiabatically transformed into a combination of forward and
backward propagating field components prior to coupling
into the photonic-crystal defect-row waveguide.16,17 The
transformation is accomplished by transitioning from a
single-mode strip waveguide into a waveguide consisting of
a one-dimensional (1D)-periodic sequence of rods (indexguided both in-plane and vertically) with a fixed period and
having the same radius as the final line-defect of rods (Stage
I). This 1D-rod waveguide is transformed into a 2D
photonic-crystal line-defect waveguide by the inclusion of
the surrounding 2D photonic crystal (Stage II). Elimination
of an abrupt photonic crystal interface avoids reflection;
hence, the transition from the 1D-rods to the line-defect
waveguide is accomplished by gradually reducing the distance between the cladding photonic crystal and the 1D-rod
waveguide [Fig. 2(a)]. The input portion of a complete device, ready for optical testing, is shown in Fig. 2(b); the SEM
image shows the input waveguide, the two-stage coupling
scheme, and the photonic crystal defect-row waveguide. The
output side of the device is the mirror of the input portion of
the device that is seen in Fig. 2(b).
Optical characterization of the devices was accomplished using a computer-controlled laser, tunable from 1430
to 1610 nm. The sample was mounted on a translation stage
capable of 10 nm resolution, and TM-polarized light was
coupled into the waveguide with a high-numerical-aperture
fiber-lens assembly. The waveguide output was first spatially
filtered, and then analyzed with a polarizing beamsplitter.
Transmission spectra were obtained by measuring the output
signal with photodetectors while tuning the wavelength of
the laser. Also, the waveguides were imaged from the top to
observe any reflection at the photonic-crystal interface and
radiation loss due to waveguide damage.
Figure 3(a) shows the transmission as a function of
wavelength measured across a 2D photonic-crystal device
having two and four rows of rods. The creation of a photonic
band gap, from 1448 to 1482 nm, is observed in the case of
four rows of rods. Time-domain simulation of the device
showed a band gap with a similar bandwidth but with a 3%
shift in wavelength. Figure 3(b) shows the measured transmission spectrum for a photonic-crystal line-defect waveguide that utilizes the aforementioned two-stage coupling
scheme on both the input and the output. For two represen-
FIG. 3. (a) Measured transmission spectra as a function of wavelength for
photonic-crystal structures with two and four rows of rods. The structure
with four rows exhibits a band gap for the wavelength range of 1448–1482
nm. (b) Measured transmission spectra for line-defect waveguides with the
two-stage coupling scheme demonstrate guiding inside the band gap. The
measurement was reproducible for two similar devices on the same chip.
The measured structures are illustrated by the inset SEM images.
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6112
tative devices, an increase in transmission is observed within
the measured band gap, demonstrating lateral guiding of
light due to the photonic band gap rather than total internal
reflection. The spectral features obtained from the bulk photonic crystal devices and the two-stage taper devices were
very reproducible for many similar devices on the same chip.
In summary, GaAs-based dielectric rod arrays were utilized to fabricate photonic-crystal devices that allow the
propagation of 1.5 µm light. Optical transmission measurements confirmed the presence of a photonic band gap for a
photonic crystal consisting of only four rows of rods. Furthermore, a two-stage coupling scheme was employed to
couple light from a conventional waveguide to a photoniccrystal defect-row waveguide. Optical measurements demonstrated light was coupled into the line-defect waveguide and
confined laterally by the band gap.
This work was supported primarily by the MRSEC Program of the National Science Foundation under Award No.
DMR 02-13282. The authors gratefully acknowledge the
contributions from M. Mondol regarding electron beam lithography.
1
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